In the textile and knitwear industry, substantial difficulties are presently encountered in automating the procedures to be carried out on knit or woven layers to make them into semi- or wholly finished goods due to the complexity in seizing these layers whether individually or in specified numbers from stacks of cut-outs; these layers most of the time are exceedingly thin (with thicknesses often less than a millimeter) and tend to adhere to one another whereby the gripping and the separation of the upper layer or of a specific number of layers above a stack amount to exceedingly delicate operations (even when carried out manually).
Presently, there is no device which proves entirely satisfactory for this work in a reliable manner compatible with automation. Essentially there are four types of grippers:
"Velcro" or hook-and-loop type hooking tape grippers which incur the drawback that they are applicable only to certain kinds of knits (inoperative with respect to most woven fabrics) and that it is difficult to detach them from the gripped layer;
adhesive grippers providing good seizure but entailing detachment difficulties similar to those mentioned above;
roller-type grippers operating by squeezing the woven fabric or knit but suffering from the drawback that their operation is highly uncertain and that they cause deep pleats in the flexible layers;
crossed-needle grippers.
A gripper of this last mentioned type is described in French patent application No. 2,468,531 (U.S. Pat. No. 4,386,770), and in the application for certificate of addition No. 2,482,064. The flexible layer is gripped by two needles extending crosswise from a casing and penetrating obliquely into one, or several, layers of the stack. Because they cross each other, these needles retain the layer(s) which they penetrate.
While such a device is fairly effective as regards knits or woven fabrics, it is wholly unreliable as regards the thin knits or woven fabrics (several tenths of a millimeter). If, for instance, a single layer is to be seized, the needles descending into the fabric must not penetrate beyond one layer thickness, or else the layer below shall be gripped too. Considering the material flexibility and elasticity, a penetration of several tenths of a millimeter is inadequate to assure suitable gripping in spite of the crossing of the needles, and quite frequently the fabric will accidentally come off while being transferred.
Moreover, as regards this kind of gripper, many precautions must be taken in detaching the lower layer tending to adhere to the seized one which is only passably held. For that reason, the designers of these kind of grippers were led, as described in the above cited application for certificate of addition, to provide hollow needles entering the fabric as far as the immediate vicinity of the inner surface of the layer to be gripped, applying a compressed air jet to the lower layer to detach it without undue risk of losing the gripped layer. In practice such detachment is, however, of low effectiveness because the device implementing the method is comparatively costly (in particular the hollow needles cost much more than ordinary needles with similar dimensions).
Elsewhere a technique for gripping fabric has been described in German patent No. 2,002,750 and in French patent No. 2,218,417 (U.S. Pat. No. 4,008,888) whereby a curved needle is made to rotate about its center of curvature so that it will project with respect to a plate and enter the fabric which is wholly immobilized, this curved needle then issuing again from the fabric on the same side at the end of its circular motion.
When the fabric pieces are stacked one upon the other, obviously and as emphasized in these patents, the needle must in the course of its circular motion about its center of curvature project from a plate by a depth less than the thickness of the layer to be seized. Otherwise, the needle also would seize the adjacent layer which it would then penetrate. Under these conditions, only one fiber or a small number of fibers of the layer to be separated are being seized by the needle.
Where thin layers are concerned (a few tenths of a millimeter), the same drawback as met by the crossed needles is encountered here, where these crossed needles provided only slightly effective gripping with a reliability below that required for automated procedures. To the knowledge of the inventors, this type of technology using a single curved needle rotating about its center of curvature never has been applied in spite of its relative age, presumably because of this major drawback of uncertain seizure and separation of a thin layer from the others of a stack. Furthermore, it is likely that such a device frequently must cause fabric degradation by rupturing the few fibers captured by the needle.
The object of the present invention is to remedy the above cited drawbacks of the known gripper means. This invention provides a novel gripping apparatus whereby gripping is assured using a needle in conditions suitable to eliminate all of the drawbacks of the known grippers and in particular of the crossed-needle grippers and of the curved needle grippers rotating about their centers of curvature.
In particular, another object of the invention is to simultaneously achieve the three conditions essential for sound operational reliability: reliable gripping of the layer(s) to be seized, assured detachment of such layer(s) with respect to the lower layers, and safe disengagement of such layer(s) when being deposited.
Another object is to achieve these results by means of compact gripper apparatus of simply design and low cost.
Another object of the invention is directly related to the above objects (reliability, low cost and compactness) and provides a gripper and transfer machine for textile parts making it possible through several gripper means to seize and to transfer pieces of any size and shape with outstanding reliability.
The difficulties in overcoming the above cited objects can be gauged in the case of thin layers superposed in a stack, namely in that the two conditions:
firmly gripping the upper stack layer to detach it from the other layers and to transfer it in the absence of danger of losing it, and
simultaneously avoiding seizing the lower layer which in that event no longer could be detached,
appear incompatible, in particular where a needle gripper is used. Either the needle penetration is limited to a depth less than the layer thickness, and in this case the layer is seized only across the minute thickness of a few fibers, and the seizure is low in effectiveness, or else the needle is made to penetrate deeper and the lower layers are seized.
The present invention resolves the problem raised by this apparent incompatibility and relates to a process and a needle apparatus suitable to allow simultaneously firmly gripping one or more thin layers and assured detachment from the lower layer which must stay in place.